Electromagnetic devices having a system of deflection coils and a focusing coil for magnetically focused camera tubes are known. Since the video amplifiers in colour television cameras become more improved (greater signal-to-noise ratio), the resulting greater amplification can be used to pick up scenes at lower light levels, and it has been found that in magnetically focused camera tubes much hindrance is experienced from certain interference oscillations which become visible in the video signal (for example as brightness interferences).
It is the object of the present invention to eliminate or at least substantially reduce the interference oscillations in question and the invention is the result of the recognition by the inventor as to the cause of these interference oscillations. It has been found that the cause of the interference oscillations is inter alia residing in the possible excitation of the focusing coil by the line or frame flyback pulses. The excitation of the focusing coil manifests itself during the active picture duration since the interference signal reaches the target of the camera tube by capacitive and/or inductive cross-talk and becomes visible in the video signal.
Due to the very large self-inductance of the focusing coil resulting from its large number of turns (a few thousand), it has not proved possible in practice to sufficiently restrict the interference signals in the focusing coil by means of one or more damping members (damping member is to be understood to mean herein in particular a capacitor, resistor or series arrangement of a resistor and a capacitor connected parallel to the coil or to a part of the coil), while the provision of a static screening around the focusing coil also proved to be of insufficient help.